Meteorological Aspects of 100 Degree Heat Locally

As the week progresses, the models continue to hint at the possibility of a major heat wave impacting the Chicagoland area. 100 degree days in the Chicago area are extremely rare; in fact, they have accounted for less than 0.2% of the days in the past 31 years. Given the rarity of 100 degrees, some basic research was done to determine the past synoptic setups in which 100 degree days were recorded. Based on each 100 degree day recorded during the past 31 years, an average was achieved, and from that average, a composite map was created. These composite maps draw stark comparisons to current model runs (as of 28 June, 2011). In particular, current model runs seem resemble the composite maps in regards to the 500mb ridge axis, as well as location of the 500mb high. Along with that resemblance, the 925mb, and 850mb temperatures also have a similarity in their max temperature (22-24C; generally greater in model runs), as well as the orientation of the thermal ridge (west-east). Even though there seems to be a good correlation between the model runs and these past events, it would be premature to issue forecasts for 100 degree temperatures. At this current time there are far too many variables that remain in question to consider forecasting high temperatures of 100 degrees or greater. Most importantly: a lot can change in the next several days.

Figure 1: Composite map based on the mean 500mb geopotential heights from the past eighteen 100 degree days. The highlight of this figure is the placement of the ridge axis, and the center of high pressure in regard to the KLOT Forecast area.

Figure 3: Composite map, using the same data set, but instead calculates the mean 850mb temperatures from the data set. Of particular interest, is the orientation of the thermal ridge. There is a west-east orientation of this ridge with temperatures of 22-24C over northern Illinois.

Figure 4: SREF 850mb forecasted temperatures for Friday (as of 28 June, 2011). Notice the orientation of the thermal ridge is similar to that in Figure 3.

Figure 5: Composite map using the same data set, but instead calculates the mean 925mb temperatures from the data set. As seen in the 850mb map, there is a west-east orientation of the thermal ridge with temperatures ranging from 27-29 Degrees Celsius.

Figure 6: NAM 925mb Temperatures as of 28 June, 2011.

Figure 7: Is a composite map using the same data set, but instead calculates the Mean Sea Level Pressure from the data set.